The invention relates to hybrid integrated circuit transformers utilizing both printed conductors on a substrate and wire bonding loops to form primary and secondary windings about a unitary toroidal core, and more particularly to such a transformer having a square toroidal core.
Commonly assigned U.S. Pat. No. 4,103,267 "HYBIRD TRANSFORMER DEVICE", issued July 25, 1978 by Olschewski illustrates the closest prior art. The hybrid transformer device disclosed therein has been widely used by the assignee in commercially marketed hybrid integrated circuit isolation amplifiers and DC to DC converters. Although highly successful commercially, there have been several problems which reduce the yield and performance of the hybrid integrated circuits containing the toroidal hybrid integrated circuit transformer described in the Olschewski patent.
One problem is that the maximum practical number of primary and secondary windings is considerably less than desirable, and hence the primary and secondary inductances are lower than desirable. The low primary inductance requires that a relatively large primary current be driven into the primary winding, necessitating use of a more expensive driver transistor having higher current capability than would be desirable. The high current level required results in increased cost and reduced reliability of products incorporating the toroidal hybrid IC transformer of the Olschewski patent.
Another more severe problem is that when the maximum permissable number of turns are provided in the primary winding and secondary winding, so as to provide adequate primary winding inductance and/or adequate amplification of the signal applied to the primary winding, the conductors at the extreme ends of the primary and secondary winding are only 10 mils apart. This short 10 mil gap has resulted in a severe problem of electrical arcing between the primary and secondary windings. Furthermore, the maximum total number of turns, including primary winding turns and secondary winding turns, is only 39 for the toroidal ferrite cores used which have an inside diameter of 197 and an outside diameter of 375 mils. Often, it would be desirable to have considerable more than 39 total turns to achieve adequate primary winding inductance and/or adequate voltage amplification.
Despite use of a very expensive dielectrical material commercially available from Dupont under the trade name parylene, and despite use of the various DI (deionized water) rinses and alcohol cleaning steps to remove all possible ionic contamination before coating the substrate with parylene dielectric coating, product failures due to electrical arcing at the adjacent end points of the primary and second windings has been a continuing source of difficulty, limiting the manufacturing yields, increasing the cost, and limiting the voltage breakdown specifications of the DC to DC converter and isolation amplifier products using the Olschewski toroidal transformer.
It would be very desirable to provide an improved toroidal transformer that is as "process-compatible" with conventional hybrid integrated corcuit manufacturing processes as the Olschewski device, but does not require the large primary drive currents required by the Olschewski hybrid transformer, has a substantially higher manufacturing yield, lower cost, higher product reliability, much higher isolation breakdown voltages than the Olschewski hybrid-transformer.
Apparently, up to now, no one has thought of providing a square hybrid integrated circuit of the general type disclosed in the Olschewski patent, even though transformers with unitary square ferrite cores have been widely used in other applications, because there was no recognition in the art that use of a miniature square toroidal transformer could be practical and lead to a great advantage in high voltage microelectric products such as isolation amplifiers and DC to DC converters.
Instead, the state-of-the-art in hybrid integrated circuits has either been to use the round ferrite toroid hybrid integrated circuit transformer device disclosed in the Olschewski reference or to use "hand-wound" round toroidal transformer devices. The hand-wound round toroidal transformers generally have been composed of two U-shaped or E-shaped ferrite core sections serving as "bobbins" upon which the primary and secondary turns, consisting entirely of wire, have been wound. An unwieldy clip arrangement has been provided to secure the two halves of the ferrite core tightly together. Very precisely machined "half-core" end surfaces have been provided to fit precisely together to minimize gaps between the "end" portions of such "half-cores". High voltage products using such toroidal transformer devices have proven impractical due to their cost or to the above-mentioned arcing between closely spaced end portions of the primary and secondary windings. In order to achieve the above-mentioned 39 turns of the Olschewski hybrid transformer, it has been necessary to provide only 6 to 7 mil center-to-center spacing between the wire bonds forming portions of each turn. This center-to-center spacing requires that the 3 mil wide metalization strips be spaced only approximately 3 to 3.5 mils apart. Such a close spacing severely presses the present state-of-the-art, and reduces the manufacturing yields more than is really acceptable.
Square transformers have been commonly used in applications other than microelectronics, however. Frequently, square transformers have been composed of separate core sections constituting "bobbins" on which the primary and secondary windings are respectively wound. These core sections then are attached at their end points by additional core sections of the same material, forming a square toroid structure. These devices usually have been utilized in low isolation voltage applications. The square core configurations, however, have not been provided to increase the minimum spacing between conductors of the primary and secondary windings, but merely for convenience of manufacture.
It is noteworthy there have been relatively few major changes in hybrid IC fabricating techniquest in recent years. The state-of-the-art is indicated by the above-mentioned Olschewski patent, Japanese Patent Public Disclosure No. 48-22737 issued Mar. 15, 1973, U.S. Pat. Nos. 3,104,377, 3,675,095, 3,764,938, 4,188,651, U.K. Pat. Nos. 811,295 and 1,469,944 and German Pat. Nos. 1,197,561 and 2,723,363.
There is a continuing need for an improved transformer of the general type described in the Olschewski patent which provides increased inductance, and much less subject to failure due to electrical arcing between the primary and secondary windings, does not introduce new difficulties into the manufacturing process, and simplifies the prior manufacturing process.